Wheel balancer using tire stiffness and loaded wheel/tire measurements

ABSTRACT

A wheel balancer is provided that includes a shaft adapted for receiving a wheel/tire assembly and rotating a wheel/tire assembly removably mounted thereon, a motor operation connected to the shaft for rotating the shaft about its longitudinal axis, thereby rotating the wheel/tire assembly, a load roller for applying a generally radial force to the wheel/tire assembly during rotation so that loaded wheel/tire assembly measurements may be determined while the force is applied thereto and a control circuit. The control circuit is responsive the loaded wheel/tire assembly measurements and to a tire stiffness value to make a determination of a predetermined uniformity parameter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. application Ser. No.09/311,473, filed May 13, 1999 now U.S. Pat. No. 6,336,364, which is acontinuation-in-part of U.S. application Ser. No, 08/706,742, filed Sep.9, 1996, now abandoned, which is a continuation-in-part of U.S.application Ser. No. 08/594,756, filed Jan. 31, 1996, now abandoned.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO MICROFICHE APPENDIX

Not Applicable.

BACKGROUND OF THE INVENTION

This invention relates to wheel balancers and in particular to improveddrive systems, safety circuitry, and displays working in conjunctionwith said drive systems for wheel balancers.

The determination of unbalance in vehicle wheels is carried out by ananalysis with reference to phase and amplitude of the mechanicalvibrations caused by rotating unbalanced masses in the wheel. Themechanical vibrations are measured as motions, forces, or pressures bymeans of transducers, which convert the mechanical vibrations toelectrical signals. Each signal is the combination of fundamentaloscillations caused by imbalance and noise.

It is believed that the drive systems for currently available balancerscould be improved to aid in operation.

Even when a wheel/tire assembly is balanced, non-uniformity in theconstruction of the tire as well as runout in the rim can causesignificant vibration forces as the wheel rolls under vehicle load. Mosttire manufacturers inspect their tires on tire uniformity machines andgrind rubber off the tires as required to improve rollingcharacteristics of the tires. Even after this procedure, tires willoften produce vibration forces (not related to imbalance) of 20 poundsas they roll on a smooth road. To put this in perspective of balancing,a 0.8 ounce balance weight is required to produce a 20 pound vibrationforce on a typical wheel traveling at 70 mph.

Many conventional balancers also assume that the wheel/tire assemblywhich is suitably balanced under an essentially no-load condition willalso be suitably balanced when installed on the vehicle and subjected tothe substantial load represented by the weight of the vehicle. Thisassumption is not valid under all conditions. It would be preferable inmany circumstances to simulate loaded conditions to improve the resultsof the balancing operation.

SUMMARY OF THE INVENTION

Among the various objects and features of the present invention is awheel balancer with improved performance.

Another object is the provision of such a wheel balancer which iscapable of simulating loads on the wheel/tire assembly.

Other objects and features will be in part apparent and in part pointedout hereinafter.

In a first aspect of the present invention, a wheel balancer includes ashaft adapted for receiving a wheel/tire assembly, the shaft having alongitudinal axis and being rotatable about the axis so as to rotate thewheel/tire assembly removably mounted thereon, a motor operativelyconnected to the shaft for rotating the shaft about its longitudinalaxis, thereby rotating the wheel/tire assembly, a load roller forapplying a generally radial force to the wheel/tire assembly duringrotation of the wheel/tire assembly so that loaded wheel/tire assemblymeasurements may be determined while the force is applied thereto and acontrol circuit responsive at least to the loaded wheel/tire assemblymeasurements and to a tire stiffness value to make a determination of apredetermined uniformity parameter.

In a second aspect of the present invention, a method is provided usinga shaft adapted for receiving a wheel/tire assembly having alongitudinal axis and being rotatable about said axis so as to rotate awheel/tire assembly removably mounted thereon, a motor operativelyconnected to the shaft for rotating said shaft about its longitudinalaxis, thereby rotating the wheel/tire assembly, and a load roller forapplying a generally radial force to the wheel/tire assembly duringrotation of the wheel/tire assembly, the method including determiningthe loaded wheel/tire assembly measurements while the force is appliedto the wheel/tire assembly, providing a tire stiffness value for thewheel/tire assembly and determining a predetermined uniformity parameterof the tire or wheel/tire assembly at least in part from the loadedwheel/tire assembly measurements and the tire stiffness value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view illustrating a generic wheel balancersuitable for use with the present invention;

FIG. 2 is a simplified top plan view illustrating the preferredembodiment of the wheel balancer of the present invention;

FIG. 3 is a block diagram illustrating electrical circuitry of the-wheelbalancer of FIG. 1 or FIG. 2;

FIG. 4 is a simplified schematic of the electronic control circuitry ofthe balancer of the present invention;

FIG. 5 is a block diagram of motor control circuitry of the balancer ofthe present invention;

FIG. 6 is a simplified block plan view illustrating the use of thebalancer of the present invention with a load roller and variousmeasuring devices;

FIG. 7 is a schematic circuit diagram of the drive circuitry used in thepresent invention;

FIG. 7A is a schematic circuit diagram of control signal circuitry usedin the present invention;

FIG. 8 is a schematic circuit diagram of electrical braking circuitryused in the present invention;

FIG. 9 is a schematic circuit diagram of a hardware safety interlockcircuit used in the present invention;

FIGS. 10 and 10A illustrate various displays used in the presentinvention; and

FIG. 11 illustrates an additional, speed setting display used in thepresent invention.

Similar reference characters indicate similar parts throughout theseveral views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning to the drawings, FIG. 1 illustrates (in simplified form) themechanical aspects of a wheel balancer 11. Balancer 11 includes arotatable shaft or spindle 13 driven by a suitable drive mechanism suchas a direct current 0.5 horsepower electric motor M and drive belt 53(FIG. 4). Mounted on spindle 13 is a conventional quadrature phaseoptical shaft encoder 15 which provides speed and rotational positioninformation to the circuitry of FIG. 3.

During the operation of wheel balancing, at the end of spindle 13, awheel/tire assembly 17 under test is removably mounted for rotation withspindle hub 13A (FIG. 2). To determine wheel/tire assembly imbalance,the balancer includes at least a pair of piezoelectric type imbalanceforce sensors 19 and 21 (or other suitable sensors such as straingauges) coupled to spindle 13 and mounted on the balancer base 12. Forease of reference herein, sensor 19 is referred to as the “L” (Left)sensor and sensor 21 is referred to as the “R” (Right) sensor.

Turning to FIG. 2, it can be seen that the actual construction of themechanical aspects of balancer 11 can take a variety of forms. Forexample, spindle 13 can include a hub 13A against which wheel/tireassembly 17 abuts during the balancing procedure. Moreover, sensor “L,”sensor “R,” and sensor 22 need not directly abut spindle 13. Forexample, various arms or rods as shown in FIG. 2 can be used tomechanically couple the sensors to the spindle so that they are exposedto the vibrations and/or forces of the spindle.

When wheel/tire assembly 17 is unbalanced, it vibrates in a periodicmanner as it is rotated, and these vibrations are transmitted to spindle13. The “L” and “R” sensors are responsive to these vibrations of thespindle. Specifically, they generate a pair of analog electrical signalscorresponding in phase and magnitude to the vibrations of the spindle atthe particular transducer locations. These analog signals are input tothe circuitry of FIG. 3, described below, which determines the requiredmagnitudes and positions of correction weights to correct the imbalance.

Turning to FIG. 3, wheel balancer 11 includes not only the “L” and “R”sensors, and spindle encoder 15, but also a graphic signal processing(GSP) chip 23. Preferably GSP chip 23 is a Texas Instruments modelTMS34010 chip. GSP chip 23 performs signal processing on the outputsignals from the “L” and “R” sensors to determine wheel imbalance. Inaddition it is connected to and controls a display 25 which providesinformation to the user, controls motor M through motor controlcircuitry 27 described in more detail below, and keeps track of thespindle position from encoder 15. More specifically, encoder 15 is a 128count, two channel quadrature encoder which is fully decoded to 512counts per wheel revolution by GSP chip 23. Although GSP chip 23 ispreferred, it should be understood that other controller circuitry couldbe used as well.

Balancer 11 also includes manual inputs 29 (such as a keyboard andparameter input data dials) which are also connected to GSP chip 23.Chip 23 has sufficient capacity to control via software all theoperations of the balancer in addition to controlling the display. TheGSP chip is connected to EEPROM memory 31, EPROM program memory 32, anddynamic RAM (DRAM) memory 33. The EEPROM memory is used to storenon-volatile information, such as calibration data, while the GSP chipuses DRAM 33 (as discussed below) for storing temporary data.

GSP chip 23 is also connected to an ADC 35 which is preferably an AnalogDevices AD7871 type device or any other appropriate chip. ADC 35 is afourteen (14) bit A/D converter with an on-board voltage reference.

The signals from the “L” and “R” sensors 19 and 21 are supplied throughanti-aliasing circuitry 37, 39 to ADC 35. More specifically, the signalsare each fed through unity gain buffers (not shown but well known in theart), to anti-aliasing filters making up part of circuitry 37, 39.Sallen/Key type low pass Butterworth filters function well for thispurpose.

The operation of the various components described above is fullydescribed in U.S. Pat. Nos. 5,365,786 and 5,396,436, the disclosures ofwhich are incorporated herein by reference. It should be understood thatthe above description is included for completeness only, and thatvarious other circuits could be used instead. The GSP chip could bereplaced by a general purpose microcontroller, for example, with no lossof efficiency. The motor drive aspects may be better understood byreference to the simplified diagram of FIG. 4 in which the GSP chip isreplaced with a generic CPU 51 and motor M drives the wheel/tireassembly through a 5.37:1 belt drive 53. Also indicated in FIG. 4 is a30 Hz watchdog pulse supplied on a set of control lines 55 to motorcontrol drive 27. In addition, control line set 55 includes a digitalsignal of software settable duty cycle which is interpreted by the motordrive as a linear function of desired “torque.” It is also possible toachieve this function using a varying analog level, a frequencymodulated digital signal, and other such approaches, depending on thedrive input requirements. The third control line to the motor drive is adrive enable line.

The motor control drive 27 is illustrated in FIG. 5. Drive circuit 27has four drive transistors Q1, Q2, Q3 and Q4 connected as shown toprovide direct current to the windings of direct current motor Mselectively with each polarity. Specifically, transistor Q1 is connectedto supply current from a dc rectified source to one side of the windingsof the motor. When current is supplied through transistor Q1 to thewindings, the circuit is completed through the windings and transistorQ4 (and a current sensing resistor RS) to ground. This causes thewindings of motor M to be energized so as to cause rotation of thebalancer shaft in a first rotational direction. Similarly, whentransistors Q1 and Q4 are rendered non-conductive and transistors Q2 andQ3 conduct, the windings are energized in the opposite polarity. It ispreferred that the direction of rotation of motor M be controlled bypulse width modulating (PWM) current to the transistors. A duty cycle of50% causes the current to flow through motor M in both directions inequal amounts. By the use of a suitably high pulse rate, the motor hasinsufficient time to respond to the rapidly reversing currents, with theresult that the motor velocity is zero. As explained below, the dc motoractively holds the shaft at its present location. This provides, ineffect, a “detent” function for the drive circuit 27.

A duty cycle of less than 50%, on the other hand, causescounterclockwise rotation of the motor shaft. As the duty cycledecreases from 50%, the counterclockwise torque becomes stronger andstronger. Similarly, a duty cycle of more than 50% causes clockwiserotation of the motor shaft. As the duty cycle increases above 50%, theclockwise torque in turn becomes stronger and stronger. A 0% duty cycleresults in maximum torque counterclockwise, while a 100% duty cycleresults in maximum torque clockwise.

It is preferred that transistors Q1-Q4 be insulated gate bipolartransistors (IGBTs) such as those sold be International Rectifier underthe trade designation IRGPC40KD2. Other similar transistors, ortransistors having similar characteristics such as MOSFETS, could alsobe used.

If it is desired to use an AC motor, the drive system would preferablybe some type of AC vector drive, although such drives are at presentsignificantly more expensive.

Whatever drive system is used, it preferably has interface circuits 34,36, and 38 for the drive enable, “torque” input, and watchdog inputsrespectively from the CPU. These signals are supplied to a control logiccircuit 40 which performs necessary logic functions, as well asconventional deadband, and current limit functions. Circuits to performthe functions of circuit 40 are well known. The current limit functionof circuit 40 depends upon the current measured by current senseresistor RS, the voltage across which is detected by a current limitdetection and reference circuit 41.

Circuit 40 has four outputs. The first, a drive fault line DF, is usedto signal the CPU chip that a drive fault has occurred. The second andthird, labeled GD1 and GD2, supply PWM control signals to the actualgate drive circuits 43 and 45, circuit 43 being connected to the gatesof transistors Q1 and Q3, and circuit 45 being connected to the gates oftransistors Q2 and Q4. The fourth output of circuit 40, labeled SD,allows circuit 40 to provide a shutdown signal to gate drive circuits 43and 45. In addition, the shutdown signal is supplied to a transistor Q5(FIG. 7A) whose drain is connected to braking resistor RB. When theshutdown signal occurs, the drive transistors Q1-Q4 turn off and thebraking resistor gets shorted between the 390 VDC bus and ground. Thisprovides braking for the motor during a shutdown condition.

To understand the improvements, it is helpful to examine some terms.FIG. 6 shows a tire 17 with a load roller 91 pressing against it, alongwith the three contact forces which are defined as radial, lateral andtractive. Tire uniformity is a term which refers to a condition in whichsome property of a tire is not symmetric about its rotational axis.There are many uniformity parameters which can be quantified.

The root-mean-square value of radial force variation is a gooduniformity parameter to use, as shown in U.S. Pat. No. 4,702,103,because it is representative of the power produced by the tire rotatingon the vehicle as a result of force variations in the verticaldirection.

A value for the tire stiffness is required to convert rim runout intoradial force variation due to rim runout: (rim runout)(tirestiffness)=radial force variation due to rim runout. Loaded radialrunout of the wheel tire assembly can also be converted to a forcevariation value by using the tire stiffness or it can be measureddirectly as will be shown later. By subtracting the rim force variationfrom the wheel/tire assembly force variation, the tire force variationcan be obtained. By shifting the angle of the tire force variationrelative to the rim-force variation, the root-mean-square value ofwheel/tire assembly force variation can be computed at many remountangles of tire to rim. Selecting the remount angle with the lowestwheel/tire assembly radial force variation is then possible.

The first harmonic of radial force variation is believed to be the bestuniformity parameter to use to minimize wheel vibration because it alsohelps minimize the first harmonic tractive force variation. U.S. Pat.No. 4,815,004 shows how tractive force variation can be determined basedon wheel properties and rotational speed squared. Taking equation (17)in U.S. Pat. No. 4,815,004 and applying it to a vehicle moving on a flatroad at constant speed, one finds:$F_{\tau} = {\frac{I\quad \omega^{2}}{r}{\sum\limits_{i = 1}^{\infty}{{{}_{}^{}{}_{}^{}}{\cos \left( {{i\quad {\omega t}} + {\varphi \quad i}} \right)}}}}$

where Fτ is the tractive force on the tire, I is the polar moment ofinertia of the wheel/tire assembly and vehicle hub, ω is the angularvelocity of the wheel, r is the outer radius of the tire, U₁ is the ithFourier coefficient of the change in effective radius per revolution ofthe tire, t is elapsed time, and φ, is the phase shift of the ithharmonic. This equation may be used to calculate the radial and tractiveforce variations on a wheel with typical properties as illustrated bythe following example:

Wheel/tire assembly is perfectly uniform except for 0.005″ radialmounting offset on the vehicle hub

Wheel assembly weight=35 lb.

O.D. of wheel=24 inches

Polar moment of inertia of the wheel/tire assembly and vehicle hub=0.7slug ft²

Vehicle speed=75 mph, which means the wheel rotational speed is 110radians/second

Tire stiffness if 1200 lb/inch

Peak to Peak Radial force variation=(0.005″)(1200 lb/in)(2)=12.0 lb

Peak to Peak Tractive force variation=((0.7 Slugft²)(110²)(radian/sec)²/1 ft)* (0.005/12 ft)(2)=7.1 lb

Note: 90 degrees of wheel rotation occurs between peak radial andtractive forces. The combination of radial and tractive forces,therefore, is equivalent to a force vector which rotates with the tire.It is believed that the relationship between wheel/tire assembly radialand tractive force variations caused by factors other than mountingoffset used in this example is similar.

Turning to FIG. 6, there is shown a load roller 91 suitably disposedadjacent wheel/tire assembly 17 so that it may be forced into engagementwith the tire so as to measure loaded runout of the assembly. Morespecifically, load roller 91 is carried on a shaft 92 suitably journaledon an L-shaped arm 93 (only the lower limb of which is clearly visiblein FIG. 7) designed to pivot about the axis of a shaft 94. CPU 51 causesthe arm to pivot to place load roller into engagement with the tire byactuating an air cylinder 95 or an air bag actuator. Air pressure tocylinder 95 can be variably adjusted by CPU control. Air pressurefeedback is provided by a sensor 102 such as those sold under the tradedesignation MPX 5700D by Motorola Inc. The feedback enables precise loadroller forces to be generated and provides a unique safety feature inthat the CPU can detect pressure problems and remove air pressure ifneeded. Rotation of shaft 94 (specifically rotation of a magnet 94Amounted on shaft 94) is sensed by a sensor 96 such as a Hall-effectsensor such as those sold under the trade designation 3506, 3507 or 3508by Allegro Microsystems Inc. and the amount of rotation is signaled tothe CPU.

By applying a known force to the tire with the load roller and watchingthe output of sensor 96, the CPU can determine the loaded runout of thewheel/tire assembly. Specifically, CPU 51 uses the output of sensor 96to measure the runout of wheel/tire assembly 17 under the predeterminedload. To determine imbalance weight amounts which are required tocounteract the forces due to runout of the wheel, the CPU also needstire stiffness information. Stiffness information can be downloadeddirectly from another measuring device such as a shock tester (notshown), or can be manually input using the manual input device 29, orcan be recalled from a stored database. Alternatively, the CPU candetermine tire stiffness directly by sequentially applying at least twodifferent loads to load roller 91 and measuring the change indeflection. The amount of additional correction weight needed tocounteract the forces due to the loaded runout is found by the followingformula:

correction mass=loaded runout first harmonic*tire stiffness*% radialforce to counteract (radius to place correction weight)*(rotationalspeed)²

With the additional mechanisms of FIG. 6, it is possible to furtherimprove the balancing of the wheel/tire assembly. For example, bymanually inputting the load range of the tire under test, the operatorcan cause CPU 51 to adjust the force on load roller 91 to a value whichwill make the loaded runout measurement most closely agree with thevibration of the wheel when it is mounted on the vehicle. Moreover, thespeed at which the vibration is to be minimized may also be inputted toCPU 51 so that imbalance correction may be optimized for this parameteras well. Generally, that speed would be selected to be at or slightlyabove the wheel hop resonant frequency. This speed also should be closeenough to the maximum operating speed to prevent excessive correction atthe maximum speed. The amount of this correction also should have amaximum limit of 0.5 oz.

In addition, CPU 51 is preferably connected to suitable sensors 88 and97 for measuring the axial and radial runout of the inside and outsiderims of assembly 17 at the bead seats. Various sensors suitable for thetask are known. These outputs are radial and axial rim runout signals.The first harmonic of radial rim runout (both angle and magnitude) isdetermined by CPU 51 using a suitable procedure such as digitalfiltering or discrete Fourier transform (DFT). The same process can beperformed to determine axial runout for each rim. With both tire and rimroundness measurements, CPU 51 is able to compare the measured valueswith stored rim and tire runout specifications. When thosespecifications are not met, it is a simple calculation to determine aremounted orientation of the tire on the rim which minimizes the totalloaded runout. CPU 51 causes the display of such an orientation ondisplay 25, along with the residual loaded runout which would remainafter remounting. Alternatively, this information may be used tocalculate the positions and amounts of required tire grinding to correctthe loaded runout.

Since the present motor control circuitry is capable of rotating thebalancer shaft at any speed, it may, if desired, slowly rotate thewheel/tire assembly while the various runout measurements are beingtaken. If desired, such measurements may be taken over two or morerevolutions of the wheel/tire assembly, and the results averaged. Ifmeasurements over different revolutions differ by more than a presetamount, CPU 51 is preferably programmed to take additional measurements.

Since the angular position of the wheel/tire assembly is directlycontrollable with the motor control circuitry, after the minimizedloaded runout position is calculated, CPU 51 may cause the assembly toslowly rotate to that position (putting that position on the tire at apredetermined position such as twelve o'clock, for example) and thenhold that position. If a tire bead breaker is integrated with thebalancer, motor M can index the rim while the tire is held stationary bythe bead breaker, eliminating many steps of current matching proceduresinvolving a separate tire changer.

Instead of measuring deflection of the load roller 91 as describedabove, alternatively CPU 51 can use the balancer force transducers 19and 21 to measure the load applied by a rigidly mounted roller. Roller91 can be rigidly mounted, for example, by loading it with a desiredforce from an air cylinder and then locking it into place with a pawl orusing an electric motor with lead screw and nut. This measurement (knownas radial force variation) can be used to determine what correctionweights are needed to cancel out the vibrations due to this wheelassembly non-uniformity. Note that tire stiffness is not required tofind the correction weights needed to counteract the wheel's radialforce variation. Using this system, the

correction mass=first harmonic of radial force variation (radius toplace correction weight)*(rotational speed)²

If there is a difference in effective diameters in two wheels mounted onthe front of a front wheel drive vehicle, there will be a tendency forthe vehicle to steer away from a straight line when driven on a flatroad. The effective diameter of a wheel/tire assembly is the distance avehicle will advance in a straight line on a flat road when thewheel/tire assembly rotates exactly one revolution, divided by the valueof π. Differences in effective diameter as small as 0.013 inches havecaused noticeable steering problems. The output of sensor 96 shown inFIG. 7, which measures the rotational position of the load roller arm,can be used to determine a value related to the effective diameter ofthe wheel/tire assembly. An alternate method to determine the effectivediameter is by measuring the ratio of the angular rotation of the loadroller and the angular rotation of the wheel/tire assembly and thenmultiplying this ratio times the diameter of the load roller. Displayinga message to the operator pertaining to effective diameters (ordifferences in diameters) of the wheel/tire assemblies and to storedspecifications is useful.

Different vehicles are sensitive to non-uniformity in wheel/tireassemblies at different levels. For example, a medium duty truck with afirst harmonic radial force variation of 50 lb. will not be likely toreceive ride quality complaints while the same value of first harmonicradial force variation on a small automobile is very likely to producean objectionable ride. By providing means for the operator to input thevehicle model or the class of vehicle on which the wheel/tire assemblyis to be mounted, and by having stored uniformity specificationscontained in the balancer's control circuit, it is possible for thebalancer to compare the measured wheel/tire assembly's uniformityparameters to the specifications and send a message to the operator whenthe wheel/tire assembly is outside of specification. A very completelisting of hundreds of vehicle models and optional equipment packagescould be used, or a very simple class system with as few as two classes(such as car vs. truck) could be used to apply stored specifications.The use of these specifications can help the operator spend time whereit is useful and avoid wasting time and effort when the specificationsshow that a large value of a uniformity parameter is acceptable.

After measurements and computations have been made to determine thevalues of various uniformity parameters, this information can bedisplayed to the operator.

Immediately after a tire is mounted to a rim, the tire bead is notalways firmly located against the rim bead seat. This can result inerrors in imbalance measurements. An important benefit of the loadroller is that it strains the tire and causes the tire bead to seatfirmly before balancing. Additionally, the load roller provides abreak-in of the tire carcass (reducing or eliminating non-uniformitiesdue to initial construction of the tire or from the tire being deformedduring shipping and storage).

Although automatic movement to a calculated rotational position isdescribed above in connection with loaded runout, it should beunderstood that the present balancer is capable of such automaticmovement to any calculated position, such as correction weightapplication points other than the standard 12:00 o'clock position. Forexample, to mount an adhesive backed weight, the CPU causes the motor torotate the wheel/tire assembly so that the correction weight positionmatches the 6:00 position so that the operator can more easily apply theweight. The particular type of weight(s) being used are manually inputusing device 29 so that the CPU can perform the proper calculation ofcorrection weight position. In addition, a desired wheel/tire assemblyrotational position may be manually requested by manual input device 29.Alternatively, an operator may manually move the wheel/tire assemblyfrom one position at which the motor is holding the assembly to another.CPU 51 is programmed to cease holding at any given position once anangular force greater than a predetermined threshold force is applied tothe assembly, such as by the operator pushing the tire. The magnitude ofsuch a force is sensed indirectly by the CPU by examining the amount ofcurrent required to overcome the applied force and hold the wheel/tireassembly in place. Once the manual movement of the assembly stops, CPU51 controls motor M to rotate the wheel to the other balance planeweight location and hold the assembly in the new position.

CPU 51 also controls the torque applied by the motor indirectly. TheEEPROM has stored the current vs. torque characteristics of the motor Mand uses those characteristics to determine the actual torque applied.This actual torque is compared to the desired torque for any particularapplication, several of which are described below. A simple example isthe application of relatively low torque at the start of the spin, whichprevents jerking of the wheel by the balancer, followed by relativelyhigher torque to accelerate the tire to measurement speed as quickly aspossible.

Slow rotation of the wheel/tire assembly is useful in severalsituations. For example, in measuring rim runout (whether loaded orunloaded) CPU 51 can rotate the assembly 17 at a controlled slow speed(1 Hz or so). This frees both of the operator's hands so that left andright rim runout may be measured simultaneously. Slow rotation is alsouseful in tightening wing nut 101 (FIG. 2) onto shaft 13. In this modeof operation CPU 51 causes the shaft to rotate at about 2 Hz while theoperator holds wing nut 101 in place. This provides a quick spin-on ofthe wing nut. Rotation continues until the current draw indicatesresistance against further movement. Alternatively, the CPU may examinethe current vs. torque characteristics of the motor to allow theoperator to continue tightening the wing nut until a desired presettorque is reached. In yet another mode, the shaft rotates at an evenslower speed (½ Hz or so) while the operator tightens the wing nut. Thisallows the wheel to “roll” up the cone taper, instead of being shovedsideways up the taper, resulting in better wheel centering on the cone.

Although the present motor control is capable of very slow rotation andfast rotation for measurement, intermediate speeds for imbalancemeasurement are also achievable and useful. For example, a large tire(as measured by sensor 96 or as indicated by a manual input from device29) may be rotated at a speed which is somewhat slower than that usedfor smaller tires. This shortens total cycle time and also allows therotary inertia of big tires to be kept below predefined safety limits(which feature is especially useful with low speed balancing with nowheel cover). Similarly, a tire with a large imbalance may be tested ata slower speed than usual to keep the outputs of sensors 19 and 21within measurable range. This prevents analog clipping of the sensorsignals and permits accurate imbalance measurements to be taken underextreme imbalance conditions. In addition, large tires may be rotated ata slower rotary speed than smaller tires to achieve the same linearspeed (MPH) for those operators who desire to test wheel imbalance atspeeds corresponding as much as possible to highway speeds or a“problem” speed.

Since the speed of rotation can be accurately controlled, it isdesirable to perform a calibration run on the balancer in which thebalancer is automatically sequenced through a multitude of speeds. Ifbalancer resonant vibrations are detected by CPU 51 at any of thosespeeds, CPU 51 stores those resonant speeds in memory and avoids thoseresonant speeds in subsequent measurement operations on a wheel/tireassembly. The magnitude of signal from an imbalance force sensornormally increases early proportionally to the square of the rotationalspeed. The angular relationship of the signal to the balancer spindlenormally does not change significantly with rotational speed. Anydeviation from this signal/frequency relationship can be detected as aresonance.

In a similar manner, the balancer can detect that an imbalancemeasurement of a wheel is invalid by comparing each revolution's sensorreading and if the magnitude or angle changes beyond preset limits thenthe measurement is considered “bad” and the CPU changes the speed ofrotation until a good reading can be obtained.

Inasmuch as the wheel/tire assembly is permitted to be rotated in eitherdirection, the present balancer may be used to rotate in eitherdirection, as selected by the operator. In addition, if desired, theassembly may be rotated in a first direction for measurement ofimbalance and in the opposite direction during the check spin aftercorrection weight(s) are applied.

The motor control drive is illustrated in more detail in FIG. 7. Thedrive circuit has four drive transistors Q1, Q2, Q3 and Q4 connected asshown to provide direct current to the windings W of motor M selectivelywith each polarity. Specifically, transistor Q1 is connected to supplycurrent from a 390VDC source to one side of the windings of the motor.When current is supplied through transistor Q1 to the windings, thecircuit is completed through the windings and transistor Q4 to ground.This causes motor M to drive the balancer shaft in a first rotationaldirection. When rotation in the opposite direction is required,transistors Q1 and Q4 are rendered non-conductive and transistors Q2 andQ3 conduct. This causes current from the 39OVDC source to flow throughQ3 and through windings W in the opposite direction. The circuit iscompleted through transistor Q2 to ground. This causes rotation of thebalancer shaft in the opposite direction.

It is preferred that transistors Q1-Q4 be insulated gate bipolartransistors (IGBTs) such as those sold be International Rectifier underthe trade designation IRGPC40KD2. Other similar transistors, ortransistors having similar characteristics such as MOSFETS, could alsobe used.

The control signals for transistors Q1-Q4 comes from the gate andemitter outputs of corresponding gate and emitter outputs of driverchips U1-U4, which are preferably Fuji EXB-840 type hybrid circuits. Theoutputs of chips U1 and U2 are always complementary, as are those of U3and U4, so as to energize the drive transistors Q1-Q4 as describedabove. This is accomplished through common drive signals PHASEI andPHASE2 applied to the driver chips. These drive signals are generated bya PWM generator U5 under the control of the CPU 23, which therebycontrols the direction of current through motor M (and hence thedirection of rotation of the shaft), as described above. Each driver hasits own power source derived from square wave signals DRVI and DRV2applied to corresponding transformers T1-T4 associated with each driverchip.

Referring to the bottom portion of FIG. 7, it can be seen that the motorwinding current in every case flows through a sensing resistor R1 or asensing resistor R3. This current is supplied to a comparator andfiltering circuit 65 composed of four op amps U11 configured withpassive devices to provide warning signals when the current throughresistor R1 exceeds a preset amount (such as 8 amps). When a warningsignal occurs, the drive signals (labeled Phase1 and Phase2) all go low,thereby shutting off the flow of current through the motor windings. ThePWM generator also receives a TORQUE-A signal and a SHUT DOWN signal,both of which are described below. More specifically, the TORQUE-Asignal and a signal representing motor current are supplied to an op-ampnetwork 66 whose output is supplied to the PWM generator. During normaloperation, the output of network 66 controls the duty cycle of PWMgenerator U5 as commanded by the CPU 23 to control operation of themotor as described above. The SHUT DOWN signal is used to shut down themotor during an abnormal situation.

The SHUT DOWN signal is generated in the circuitry of FIG. 7A. FIG. 7Ashows a plug J2 attached to the CPU 23 (the CPU is not shown in FIG. 7A)which supplies the desired torque information, and the watchdog andenable pulses, from the CPU to the circuitry of FIG. 7A. The plug alsopasses back to the CPU a fault signal. The desired torque, watchdog andenable signals are passed through optical isolators OP1-OP3 to theremaining circuitry. In similar fashion, the fault signal is opticallyisolated by unit OP4.

The desired torque signal is converted by a circuit 78 to analog form,with the corresponding analog signal being labeled TORQUE. The desiredtorque signal is also supplied to a multivibrator circuit 80, whoseoutput is an indication of whether or not the desired torque signal isbeing received from the CPU. This is ORed with other signals, andsupplied through an inverter 82 to a flip-flop 84, whose output is theSHUT DOWN output. The enable signal is supplied directly from isolatorOP2 to the enable pin of flip-flop 84, so that when the enable signalfrom the CPU is missing, the SHUT DOWN signal is active.

The watchdog signal is supplied to a multivibrator circuit 86, whoseoutput is also supplied through the inverter 82 to flip-flop 84. Thefinal ORed input to the flip-flop is an OVERSPEED signal, describedbelow. As can be seen, when any of the control signals indicate aproblem, the SHUT DOWN signal represents that fact. This signal issupplied directly to the PWM generator U5 (FIG. 7) to shut down themotor.

Turning to FIG. 8, there is shown an alternative circuit for providing adynamic braking function. Specifically, if power is removed frombalancer 11 during operation, the dc motor M functions as a generator solong as the wheel/tire assembly continues rotating. This keeps the dcbus alive during the dynamic braking process. The braking circuitincludes a 33 ohm, 50 watt resistor R15 connected between the 390-voltsource and a transistor Q9. When the transistor conducts, resistor R15serves to dissipate the energy in the rotating motor, bringing it to ahalt. Transistor Q9 conducts when the back emf of the motor rises abovea threshold. This can occur during two situations: normal motordeceleration and power loss. The motor is normally decelerated byapplying a reverse torque to the motor using the H-bridge describedabove. This causes the back emf to rise. During deceleration, thetransistor Q9 is pulsed to keep the bus at a nominal level duringreverse torque braking. During power loss, the transistor is heldfull-on, thereby providing electric braking.

FIG. 9 illustrates a hardware safety interlock circuit. In this circuit,various signals (such as hood open signals, and rotation rate signals(labeled CHA and CHB)) are supplied to an independent 8051-typeprocessor U21. When the encoder signal represent a rotational speedabove a preset limit (such as 20 to 30 rpm) and the hood is open, chipU21 provides an overspeed signal through an opto-coupler OPT11 and atransistor Q21 to the connection labeled OVERSPEED on FIG. 9. This, asdescribed above, is used to shut down the motor by controlling theoperation of PWM generator U5.

Similarly, when the inputs indicate an excessive torque situation (e.g.,over 2-3 ft-lbs.) When the hood is open, chip U21 signals this conditionthrough an opto-coupler OPT13 which controls the output of a 4053-type1-of-2 switch U23. Switch U23 also provides the regular “Torque” signal(described above in connection with FIG. 7A) to the rest of the controlcircuitry when the hood is down. When the hood is up, switch U23connects the TORQUE input to a ⅓ voltage divider, which thereuponsupplies a signal through a voltage follower to the TORQUE-A output,which is supplied (FIG. 7) to the drive circuit to limit the torque to apreset amount (2-3 ft-lbs.). When the hood is down, the TORQUE input issupplied directly through the voltage follower.

Turning to FIG. 10, there is shown an improved display 25B. As describedabove, the present balancer can acquire the loaded runout, axial runoutsand radial runouts of the wheel/tire assembly. These are displayed inconnection a three-dimensional representation of the wheel/tireassembly. Specifically, the display of FIG. 10 represents an example ofthe runout display after the spin has determined loaded runout and afterthe runout arms 88 and 97 have been used and retracted. The CPUtranslates the runouts and force variations obtained at the devicesparticular contact points to 12:00 position runouts, and displays theacquired runouts with respect to the instantaneous position of the mainshaft encoder as if the user had taken the time and expense to placerunout gauges on the physical wheel.

The display of FIG. 10 shows the total indicated readings of runout (bymeans of the numerals on the displayed needle gauges 111, 113, 115,117), any bad total readings (by highlighting the corresponding numeralsin a contrasting color), and the graphical range of the runout readings(by providing a lighter colored pie section in each needle gaugerepresentation corresponding to the measured variation in runout). Thislatter feature allows the user to tell at a glance the total travel theneedle of each gauge would have without rotating the wheel at all. It ispreferred that the gauge representations have green, yellow and redcolor bands, which are automatically scaled per the sensitivity of thatparticular reading for that particular type of vehicle.

Note that the display includes bumps on the rim and tire. Theserepresent the relative magnitudes and locations of the measured runouts.These features move around the axis of the displayed wheel as the actualwheel is moved. The display also includes a representation 121 of theposition of the valve stem on the display. This position is acquired bythe system via encoder 15. For example, the user can be instructed tostart the measurements, with the rotational position of the valve stemat the 12:00 o'clock position.

In the display of FIG. 10, the loaded runout “high spot” is nearlyopposite the rim high spots, as can be seen readily from the display.This means that matching of the tire to the rim by removing the tire andrepositioning it could greatly reduce or even eliminate total runout.The system is programmed to respond to the “Show after Optimized” switch125 to illustrate the various runouts, which would result if thismatching were performed, thereby informing the user if the procedurewould be worthwhile.

The various key displays on FIG. 10 (Show After Optimized key 125, Exitkey 127, Measure Rim Runouts 129, and Show Before Optimized key 131) canbe replaced by the key displays shown in FIG. 10A to allow the user torequest additional functions as indicated by those displays. The showT.I.R. Readings (total indicated readings) is the default.Alternatively, live readings as obtained from the data acquisitionsystem may be displayed, as may be the tolerance values for the totalindicated readings for the measurement for the selected vehicle. TheRotate to Next Position key can be used to signal the motor to positionand hold the wheel/tire assembly at the various high spots for thepurpose of applying indicator marks to the assembly.

If sensor 88 or 97 is pulled away from its home position while therunout screen of FIG. 10 is displayed, the balancer turns that sensorinto a virtual dial indicator. By placing the sensor against the rim, akey (not shown) can be pressed to zero the corresponding gauge display,just like a real dial indicator. Then, as the wheel/tire assembly isturned, the gauge display shows the runout as it is measured, just likea real dial indicator.

Turning to FIG. 11, there is shown a display of the present balancerwhich allows the user/operator to manually set the desired speed atwhich the balancing is to occur. This feature is useful, for example,when the vehicle owner complains of a vibration at a particular speed,such as 30 mph. To test the balance of the wheel/tire assembly at 30mph, the operator presses soft key 141, labeled “Velocity Mode”, whichcauses the display of the simulation of a vehicle dashboard 143 as shownin FIG. 11. The operator can use a soft key 145 to select either thelinear speed (e.g., the complained of 30 mph) or the actual rotationalspeed in revolutions per minute by toggling key 145. Soft keys 147, 149can then be used to set the linear speed or rpm as desired. As theselected speed is changed, the dashboard display changes accordingly.Once the desired speed is reached on the display, the operator usesanother soft key (not shown) to initiate the actual balancing procedure.As the balancer starts accelerating the wheel/tire assembly, preferablythe dashboard display shows the corresponding vehicle speed, so that theoperator (and customer) can verify that the balance is tested at thedesired speed.

In the event the operator selects a linear speed, the CPU 23 convertsthe selected linear speed to the corresponding revolutions per minutefor that particular wheel/tire assembly. Whether linear speed or rpm isselected, CPU 23 is responsive thereto to cause the motor to rotate thewheel/tire assembly at the desired speed. In this way, the operator caninput a desired speed and balancer tests the wheel/tire assembly at thatspeed.

A knob 159 is disposed adjacent display 25. Knob 159 is used to enter adesired force to be applied to the wheel/tire assembly by load roller 91during the balancing procedure. For example, the operator may wish totest the wheel/tire assembly under normal operating conditions, whichwould involve applying a force which corresponds to the weight normallyapplied to that particular wheel for that particular vehicle. To do thisthe knob is turned as needed to change the numerals 161 displayedadjacent knob 159 until they reach the desired value. Altematively, ifthe vehicle type has already been entered into the system, the CPU 23can preset the load to be applied once the axle on which the wheel/tireassembly is to be mounted is identified.

In view of the above, it will be seen that all the objects and featuresof the present invention are achieved, and other advantageous resultsobtained. The description of the invention contained herein isillustrative only, and is not intended in a limiting sense.

What is claimed is:
 1. In a wheel balancer, the improvement comprising:a shaft adapted for receiving a wheel/tire assembly, said shaft having alongitudinal axis and being rotatable about said axis so as to rotatethe wheel/tire assembly removably mounted thereon; a motor operativelyconnected to the shaft for rotating said shaft about a longitudinal axisof the shaft, thereby rotating the wheel/tire assembly; a load rollerfor applying a generally radial force to the wheel/tire assembly duringrotation of said wheel/tire assembly so that loaded wheel/tire assemblymeasurements may be determined while the force is applied thereto; acontrol circuit responsive at least to the loaded wheel/tire assemblymeasurements and to a tire stiffness value to make a determination of apredetermined uniformity parameter.
 2. The wheel balancer as set forthin claim 1 wherein the tire stiffness value is provided by a measuringdevice separate from but electrically connected to the balancer.
 3. Thewheel balancer as set forth in claim 1 wherein the tire stiffness valueis provided by a suspension tester.
 4. The wheel balancer as set forthin claim 1 wherein the tire stiffness value is provided by a manuallyoperable input device.
 5. The wheel balancer as set forth in claim 4further including a stored database of tire stiffness data, saidmanually operable input device being actuable by an operator to select adesired tire stiffness value from said database.
 6. The wheel balanceras set forth in claim 1 wherein the tire stiffness value is determinedby the control circuit from a change in position of the load rollerresulting when forces of different magnitudes are applied to thewheel/tire assembly by the load roller.
 7. The wheel balancer as setforth in claim 1 wherein said control circuit is responsive to a tireparameter to adjust the force applied by the load roller.
 8. The wheelbalancer as set forth in claim 1 wherein the loaded wheel/tire assemblymeasurements are measurements of loaded radial runout or variation inradial force.
 9. The wheel balancer as set forth in claim 1 includingstored wheel/tire assembly uniformity specifications, the controlcircuit being programmed to display a message to an operator if measureduniformity is outside of the specifications.
 10. The wheel balancer asset forth in claim 1 further including a feedback sensor to measure theforce generated by the load roller.
 11. The wheel balancer as set forthin claim 1 wherein the predetermined uniformity parameter is forcevariation.
 12. The wheel balancer as set forth in claim 1 wherein thecontrol circuit determines correction weight magnitudes and positionsfor correcting an effect of the predetermined uniformity parameter. 13.The wheel balancer as set forth in claim 1 further including a sensorfor measuring runout of a wheel rim of the wheel/tire assembly at a beadseat of said wheel rim, said control circuit being responsive to themeasurements of wheel rim runout, and responsive to measured loadedradial runout of the wheel/tire assembly to determine an angular remountposition of a tire on the rim to minimize a predetermined uniformityparameter of the tire or wheel/tire assembly.
 14. The wheel balancer asset forth in claim 13 further including a display to indicate to a usersaid angular remount position of the tire with respect to the rim. 15.The wheel balancer as set forth in claim 14 wherein the control circuitcontrols the display to indicate the value the uniformity parameterwould have if the tire were mounted to the rim at said angular remountposition.
 16. The wheel balancer as set forth in claim 1 wherein theload roller is adapted to move radially during determination of loadedrunout.
 17. In a wheel balancer having a shaft adapted for receiving awheel/tire assembly, said assembly including a wheel and a tire mountedthereon, said shaft having a longitudinal axis and being rotatable aboutsaid axis so as to rotate the wheel/tire assembly removably mountedthereon, a motor operatively connected to the shaft for rotating saidshaft about a longitudinal axis of said shaft, thereby rotating thewheel/tire assembly, and a load roller for applying a generally radialforce to the wheel/tire assembly during rotation of said wheel/tireassembly, a method comprising: determining loaded wheel/tire assemblymeasurements while the force is applied to the wheel/tire assembly;providing a tire stiffness value for the wheel/tire assembly;determining a predetermined uniformity parameter of the tire orwheel/tire assembly at least in part from the loaded wheel/tire assemblymeasurements and the tire stiffness value.
 18. The method as set forthin claim 17 wherein the tire stiffness value is provided by a measuringdevice separate from but electrically connected to the balancer.
 19. Themethod as set forth in claim 17 wherein the tire stiffness value isprovided by a suspension tester.
 20. The method as set forth in claim 17wherein the tire stiffness value is provided by a manually operableinput device operated by a user.
 21. The method as set forth in claim 20further including a stored database of tire stiffness data, saidmanually operable input device being actuable by an operator to select adesired tire stiffness value from said database.
 22. The wheel balanceras set forth in claim 17 wherein the tire stiffness value is determinedfrom a change in position of the load roller resulting when forces ofdifferent magnitudes are applied to the wheel/tire assembly by the loadroller.
 23. The method as set forth in claim 17 wherein a controlcircuit output is adjusted in response to a tire uniformity parameter.24. The method as set forth in claim 17 including measuring wheel/tireassembly uniformity and comparing stored wheel/tire assembly uniformityspecifications with the measured uniformity.
 25. The method as set forthin claim 17 further including measuring the force generated by the loadroller.
 26. The method as set forth in claim 17 including the step ofdetermining correction weight magnitudes and positions for correctingdetermined imbalance.
 27. The method as set forth in claim 17 furtherincluding measuring runout of a wheel rim of the wheel/tire assembly ata bead seat of the wheel rim, and determining an angular remountposition of the tire on the rim to minimize some predetermineduniformity parameter of the wheel/tire assembly.
 28. The method as setforth in claim 27 further including indicating to a user said angularremount position of the tire with respect to the rim.
 29. The method asset forth in claim 27 further including displaying to a user the valuethe uniformity parameter would have if the tire were mounted to the rimat said angular remount position.
 30. The method as set forth in claim17 wherein the load roller is adapted to move radially during thedetermination of loaded wheel/tire assembly measurements.